Add offsets_from_biluo_tags helper and tests (see explosion#1626)

spacy/gold.pyx

@@ -541,5 +541,24 @@ def biluo_tags_from_offsets(doc, entities, missing='O'):
     return biluo
 
 
+def offsets_from_biluo_tags(doc, tags):
+    """Encode per-token tags following the BILUO scheme into entity offsets.
+
+    doc (Doc): The document that the BILUO tags refer to.
+    entities (iterable): A sequence of BILUO tags with each tag describing one
+        token. Each tags string will be of the form of either "", "O" or
+        "{action}-{label}", where action is one of "B", "I", "L", "U".
+    RETURNS (list): A sequence of `(start, end, label)` triples. `start` and
+        `end` will be character-offset integers denoting the slice into the
+        original string.
+    """
+    token_offsets = tags_to_entities(tags)
+    offsets = []
+    for label, start_idx, end_idx in token_offsets:
+        span = doc[start_idx : end_idx + 1]
+        offsets.append((span.start_char, span.end_char, label))
+    return offsets
+
+
 def is_punct_label(label):
     return label == 'P' or label.lower() == 'punct'

spacy/tests/gold/test_biluo.py

@@ -1,7 +1,7 @@
 # coding: utf-8
 from __future__ import unicode_literals
 
-from ...gold import biluo_tags_from_offsets
+from ...gold import biluo_tags_from_offsets, offsets_from_biluo_tags
 from ...tokens.doc import Doc
 
 import pytest
@@ -41,3 +41,14 @@ def test_gold_biluo_misalign(en_vocab):
     entities = [(len("I flew to "), len("I flew to San Francisco Valley"), 'LOC')]
     tags = biluo_tags_from_offsets(doc, entities)
     assert tags == ['O', 'O', 'O', '-', '-', '-']
+
+
+def test_roundtrip_offsets_biluo_conversion(en_tokenizer):
+    text = "I flew to Silicon Valley via London."
+    biluo_tags = ['O', 'O', 'O', 'B-LOC', 'L-LOC', 'O', 'U-GPE', 'O']
+    offsets = [(10, 24, 'LOC'), (29, 35, 'GPE')]
+    doc = en_tokenizer(text)
+    biluo_tags_converted = biluo_tags_from_offsets(doc, offsets)
+    assert biluo_tags_converted == biluo_tags
+    offsets_converted = offsets_from_biluo_tags(doc, biluo_tags)
+    assert offsets_converted == offsets